Negative Hydrogen Ion Beam Research Articles

Production of negative ion beams through charge transfer between negative hydrogen ion beams and non-metallic gases

Negative ion beam applications in tandem accelerators are used for nuclear research, environmental studies, materials analysis, medical treatments, and ion implantation in semiconductor devices. Conventional methods for generating negative ions for tandem accelerators rely on metallic vapors (typically alkali) for charge exchange, which pose challenges like contamination, electrical shorting and breakdowns, and maintenance issues. To address these drawbacks, this work explores an alternative approach to produce negative ions using a non-metallic charge exchange process. It involves directing negative hydrogen ions into neutral gases within a specially designed charge exchange cell equipped with an electrostatic accelerator. The method is applied to various gas targets, including He, H2 and O2, to accelerate and measure resulting negative ions. This innovative approach aims to mitigate contamination concerns associated with metallic vapor double-charge exchange methods and explore novel avenues for negative ion production through charge transfer. Any newly formed negative ion beam current conversion ratios from the incident H− beam will be reported as progress in this research.

Plasma homogeneity over the extraction beamlet groups at the half size ITER negative ion source at ELISE test facility

The negative ion source for neutral beam injection at ITER has to provide high-intensity and low-divergence negative hydrogen and deuterium ion beams. Extracting the negative ions from the plasma is inevitably accompanied by co-extraction of electrons, which can limit the source performance, especially in deuterium. Reducing the co-extracted electrons is done by applying a magnetic filter field. On the one hand, the filter field reduces the electron temperature and the amount of co-extracted electrons, on the other hand, it introduces × B→ drifts. The drifts create a vertical plasma inhomogeneity in front of the extraction area and, consequently, an asymmetry of the co-extracted electrons. This motivated detailed studies on the vertical plasma inhomogeneity over the beamlet groups in the ELISE ion source using a movable Langmuir probe in deuterium plasma. It is demonstrated that the vertical distribution of the plasma potential changes together with the sheath, forming at the plasma grid – from electron repelling to attracting sheath. A repelling sheath reduces the flux of electrons from the plasma to the grid surface and consequently, more electrons are co-extracted. The attracting sheath collects the majority of electrons on the grid and reduces the co-extracted electrons.

Simple 3D PIC Analysis for Beam Phase Space Oscillation in RF Driven Negative Hydrogen Ion Source

Temporal oscillation of the negative hydrogen ion (H−) beam in the Radio Frequency (RF) ion source is investigated by a simple 3D3V Particle-In-Cell (PIC) model. The model solves electron, proton, and H− transport processes in the extraction region. The calculation domain is from the vicinity of the beam aperture to the ground electrode. To understand the relation between the plasma density oscillation and the extracted H− beam characteristics, the input electron and proton fluxes from the driver region are varied parametrically with the fundamental J-PARC RF frequency (2 MHz). The numerical results give an idea of the main physical processes between the oscillations of the plasma parameters and the extracted H− ion trajectories depending on the plasma meniscus shape in the different RF phases.

Sequential Computer Simulation of H-minus Ion Dynamics from Source to RFQ Exit with Beam Rotation

An end-to-end simulation of the beam dynamics in the system of extraction, beam transportation and the first section of the accelerator (RFQ) is carried out. The proposed method of modeling the dynamics of ions from the source to the exit from the RFQ made it possible to quickly and efficiently change the parameters of the matching system depending on the final results of the beam passing into the RFQ. A system for matching a beam of negative hydrogen ions has been developed and optimized, which makes it possible to rotate the beam to block the propagation of cesium into the accelerator channel and to match the beam with RFQ. As a result of optimization of the matching channel, it is possible to obtain an emittance increase in RFQ of no more than 25 % for 90 % of the beam fraction, while the capture of beam particles in the acceleration mode is 98 % of the injected beam. The increased drifts in the matching system leave enough space for the placement of diagnostic equipment, a vacuum system and a beam correction system. The dependence of the beam dynamics in the RFQ on the charge distribution over the beam cross section at the output of the matching channel is established.

Study of the Method of Photon Neutralization of Powerful Beams of Negative Ions at the Budker Institute of Nuclear Physics

A short review of the studies carried out at the Budker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences (BINP SB RAS) on the photon neutralization of the beams of negative ions is presented. The principal distinctive feature of the presented approach consists in the nonresonant accumulation of photons in a limited space. Their confinement is based on the adiabatic motion of photons in a system of concave mirrors, which is insensitive to the quality of the injected radiation. An analysis is carried out of the possibility of using the neutralizer based on such a nonresonant photon trap in large-scale installations such as ITER and TRT, and a future experiment is described on the photon neutralization using a beam of negative hydrogen ions with energy up to 130 keV and a current of about 10 mA.

Beam Profile Measurement Using the Highly-Oriented Pyrolytic Graphite

The mitigation of heat loading is one of the important issues for beam instrumentation to measure the high-power proton beam. Recently, the highly-oriented pyrolytic graphite (HOPG) material was used for the target probe of the bunch-shape monitor at the front end in the Japan Proton Accelerator Research Complex (J-PARC). Since the thermal conductivity of the HOPG is high, it is suitable to measure both transverse and longitudinal beam profiles under the condition of high heat loading. As an application of the HOPG, for example, the thin HOPG may be used as a substitutive material of the target wire for the transverse profile monitor such as the wire scanner monitor. The possibility of the HOPG target for both transverse and longitudinal beam profile monitors is discussed from some results of the test experiment using the 3 MeV negative hydrogen ion beam at the test stand.

Final design and manufacturing of the neutralizer for the negative ion-based neutral beam injector test facility of CRAFT

The Comprehensive Research Facility for Fusion Technology (CRAFT) is a highly integrated research and development base. It consists of 20 different demonstrating or testing facilities that address most of the critical technologies and systems for the fusion reactor in China. A negative ion-based neutral beam injector (NNBI) test facility is established in CRAFT. A gas neutralizer is applied to the CRAFT NNBI test facility for the neutralization of the negative hydrogen ion beam. The target of the neutralization efficiency is more than 50%. The length of the neutralizer is 3 m. The neutralizer is divided into two adjacent vertical channels and the total opening is 1.7 m × 0.32 m. The neutralizer is mainly composed of channel plate, support structure, beam scraper, cooling water pipes, and gas inlet pipes. Based on the design requirements of the neutralizer, the support structure and cooling water pipes were optimized, to reduce the total weight. The structural strength of all components was simulated and compared to the preliminary design. The manufacturing of the neutralizer is ongoing. Some manufacturing challenges have been overcome, such as deep hole drilling. Several 1.7 m long cooling channels were successfully drilled in a copper panel and the ratio of depth to diameter was 94.

Measurement of the longitudinal bunch-shape distribution for a high-intensity negative hydrogen ion beam in the low-energy region

A bunch-shape monitor (BSM) is a useful device for performing longitudinal beam tuning using the pointwise longitudinal phase distribution measured at selected points in the beam transportation. To measure the longitudinal phase distribution of a low-energy negative hydrogen (${\mathrm{H}}^{\ensuremath{-}}$) ion beam, highly oriented pyrolytic graphite (HOPG) was adopted for the secondary-electron-emission target to mitigate the thermal damage due to the high-intensity beam loading. The HOPG target enabled the measurement of the longitudinal phase distribution at the center of a 3-MeV ${\mathrm{H}}^{\ensuremath{-}}$ ion beam with a high peak current of about 50 mA. The longitudinal bunch width was measured using HOPG-BSM at the test stand, which was consistent with the beam simulation. The correlation measurement between the beam transverse and longitudinal planes was demonstrated using HOPG-BSM. The longitudinal Twiss and emittance measurement with the longitudinal Q-scan method was conducted using HOPG-BSM. The measured longitudinal emittance is consistent with the beam simulation using the radio-frequency quadrupole design input.

An integration design model for a large-scale negative ion accelerator of neutral beam injection system for fusion application

Design processes of a large-scale negative ion accelerator for neutral beam injection (NBI) application involve a series of physics and engineering issues, which include high voltage holding, background gas and stripping losses, beamlets optics and steering, particle and power flux on the grid electrodes, heat removal, thermal deformation, and stress of the grids. A self-consistent design model covering all these critical issues has been developed, where the results of one design aspect can be directly plugged into another one as the input conditions with little approximation or assumption. This design model has been applied to the negative ion accelerator of the NBI test facility of CRAFT (Comprehensive Research Facility for Fusion Technology), which is designed to produce a negative hydrogen ion beam of 25 A with the particle energy of 400 keV and the pulse duration of 3600 s. The accelerated current density is required to be 210 A/m2 from 768 apertures with a diameter of 14 mm. The evaluated results of the CRAFT accelerator design are quantitatively analyzed. Additionally, the modeling is applied to a large-scale and relatively complete structure of the multi-grid electrodes. Hence, some nonuniformities or special distributions appear in different design issues, which were not noticed in the reference works.

ASSESSMENT OF THE POSSIBILITY OF MODERNIZING A SPACE CHARGE LENS FOR FOCUSING BEAMS OF NEGATIVE HYDROGEN IONS DUE TO CONFINEMENT OF POSITIVE BACKGROUND IONS BY AN ALTERNATING ELECTRIC FIELD

The work is devoted to considering the possibility of improving the focusing properties of a space charge lens due to charge retention of positive ions by an electric field that varies both in time and along the axis of the system. Numerical calculations of motion trajectories of positive argon ion during interaction with an alternating electric field with oscillation frequencies in the range of 1...8 MHz and amplitude in the range of 1.5...6 kV were carried out. A significant increase in the residence time of positive argon ion in the overcompensated beam of negative hydrogen ions is shown, which indicates the possibility of significant influencing focusing charge in the lens for focusing beams of negative hydrogen ions.

SPIDER Beam Homogeneity Characterization Through Visible Cameras

High energy beams of negative hydrogen and deuterium ions are needed to heat and sustain the plasma of future nuclear fusion reactors, in particular, in the experimental reactor International Thermonuclear Experimental Reactor (ITER). Besides the beam energy, low divergence (< 7 mrad) and homogeneity (greater than 90%) are required, as well as a beam current of 285 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathbf {A/}\mathbf {m}^{\mathbf {2}}$ </tex-math></inline-formula> in deuterium and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$350~\mathbf {A/}\mathbf {m}^{\mathbf {2}}$ </tex-math></inline-formula> in hydrogen. Source for the production of ions of deuterium extracted from a radio-frequency plasma (SPIDER), the full-size prototype of the ITER negative ion source, is equipped with a tomographic system consisting of 15 visible cameras, which observe the light produced by the interactions of the beam particles with the background gas after the accelerator, all around the beam itself, allowing a complete characterization of the beam shape and intensity. In fact, when the beam particles propagate in the background gas, they emit light in the visible range, due to the production of excited neutrals and ionization of the background particles. This light allows studying the beam properties since it is proportional to the beam current density itself. In the SPIDER ion source, magnetic and electric fields are used to optimize the beam current density, by reducing the electron temperature and density close to the extraction region. Also, cesium is evaporated in the plasma as a catalyst of negative ion production. In this work, the impact of these fields and of the cesium presence on beam properties will be discussed by means of visible tomography, using both the 1-D beam profiles and the 2-D tomographic reconstructions.

Effect of cathode shape on the lifetime of arc discharge negative Penning ion source

Internal Penning ion source for the production of negative hydrogen ions is commonly used in various compact cyclotrons. The ion source generally operates at high power DC discharge to provide a high negative hydrogen ion beam current. In this case, the cathode will undergo intense ion sputtering and evaporation due to fierce ion collision. Then, the sputtered material from the cathode will deposit on the inner wall of the ion source discharge chamber. These deposits may lead to slight changes in the structure of the discharge chamber and affect the ion beam extraction stability. Additionally, these deposits would accidentally break and spall, inducing sudden changes in the electric field distribution or even short circuits of the discharge. As a result, the working life of the ion source is dramatically declined. Thus, the relationship between the cathode size and the plasma state was theoretically explored in this study to prolong the ion source’s lifetime. The theory suggests that the impact power of ions on the cathode can be significantly reduced when the cooling capacity of the cathode is reduced. Meanwhile, the cathode temperature is slightly increased, and the discharge voltage declines. Experiments of 4.8 mm and 6 mm in diameter of the cathode were conducted by changing the diameter of the cathode to influence the radiation cooling efficiency. Specifically, the beam current difference was within ±15% before and after changing the cathode size in the same discharge current when the small cathode was used; the beam current of the small cathode was more than 10% (even 40%) better than that of the big one at the same operating power. Before the ion source short-circuited at the same discharge current, the small and big cathode groups operated 5536 μAh and 3000 μAh, respectively. Another test demonstrated that after working for 50 h at 1.8 A discharge current, the mass loss of small cathode was 0.0456 g, and the mass loss of big cathode was 0.0798 g.

Two-dimensional spatial distribution and production mechanism of H− ions in cylindrical inductively coupled H2 discharges

In this work, radio frequency (RF) inductively coupled hydrogen plasmas have been investigated using a two-dimensional (2D) hybrid model, consisting of a fluid module and an electromagnetic field module. This paper focuses on the effects of driver chamber radius, RF power, and gas pressure on the spatial distribution of H− density. The results show that H− ions are produced all over the driver chamber and exhibit a strong accumulation in the plasma center. Furthermore, this accumulation becomes much stronger with a smaller radius, due to the larger dc electric field intensity. While the H− density in the plasma center first increases and then decreases with the increasing radius. This is because the total production rate of H− has an opposite tendency to that of the dc electric field intensity. Therefore, the spatial distribution of the H− is governed by the dc electric field and the production mechanism. In addition, the accumulated area of H− at different powers is nearly invariable, whereas it spreads out with pressure. This can be attributed to the different spatial profiles of the dc electric field. To validate our model, the calculated electron density is compared with the experimental result, and a reasonable agreement is achieved. Hopefully, the results in this work could lead to a deeper insight into the variations of the spatial distribution of H− density on the driver chamber radius and discharge conditions, which is very important in the design of a high efficiency and compact negative hydrogen ion beam source.

Classical analytical description of negative hydrogen ions

Studies of negative hydrogen ions (NHI), such as, H– and D–, are important from both practical and fundamental points of view. One of the reasons for their practical importance has to do with heating tokamak plasmas by external beams of NHI. Another practical use is the injection of NHI in high-power proton accelerator facilities to enhance their performance. In addition, NHI are also important for studies of opacities of the atmospheres of the Sun and of A-type stars. We present a classical analytical description of NHI in the situation where the underlying hydrogen (or deuterium) atom constitutes a rapid subsystem while the outer electron represents a slow subsystem. We focus on the case where the inner electron is in a circular state, so that the subsystem “nucleus plus inner electron” does not have the average electric dipole moment — in distinction to previous studies where the presence of the average dipole moment was the crucial requirement. By using the separation of rapid and slow subsystems, we show analytically that there is a classical bound state in such system and studied its parameters. In particular, we calculate analytically the primary frequency of the radiation of such system. This could be used for its experimental detection. The states that we found for the above systems could be considered as classical counterparts of the double-excited states of NHI previously studied in the literature in frames of quantum mechanics. The existence of classical counterparts of the double-excited states of NHI is a counterintuitive result.

Characteristics of extracted ion beam from a cesium-free negative ion source using sheet plasma.

We report the characteristics of extracted beam current in the test plasma produced by direct current for sheet plasma upgrade producing negative ions by volume production without cesium (Cs) seeding. The negative hydrogen ion beam is extracted by a two-grid extraction system, which is located at the periphery of the sheet plasma. Experimental observations show that (i) negative hydrogen ions are successfully extracted from the sheet plasma by single/multi-aperture grids and (ii) the ratio of the extracted electron current IEG(e) and the hydrogen negative ion current IEG(H-), IEG(e)/Ic(H-), decreases from 8.0 to 2.0 with an increase in the height of the electron fence (HEF), which is a filter that prevents electron diffusion from the extraction region.

Negative ion beam extraction in volume mode on the RF negative ion source at ASIPP

• A test equipment of RF negative ion source was modified according to the commissioning results of negative ions extraction. • The negative ion beam current density without Cs seeding was increased to 25 A/m 2 at 50 kW RF power. • The extracted H- current tended to be saturated at high RF power, but the co-extracted electron current kept increasing. • The e/H- ratio is monotonously decreased with the rising bias voltage. Hefei utility negative ions test equipment with RF source (HUNTER) has been developed at the Institute of Plasma Physics Chinese Academy of Sciences (ASIPP), in order to study the RF negative ion source for neutral beam injection application. This paper reports on several recent modifications made to HUNTER, including magnetic confining field, magnetic filter field, extraction area, pumping system, RF generator, high voltage power supply. And then, the performance of negative hydrogen ion beam extraction in volume mode without Cs seeding was reviewed and compared to the former commissioning results. The current of extracted negative ions and co-extracted electrons were recorded by the Hall current sensors installed on the electric wires connecting to the extraction grid, and ground grid and calorimeter. A similar extraction experiment from argon plasma verified the effective deflection and absorption of co-extracted electrons on the extraction grid, where the leaking electrons were less than 2%. The characteristic of negative ion beam extraction in volume mode was analyzed in different RF power, extraction voltage, bias voltage. The extracted negative ion density of 25 A/m 2 was achieved and the corresponding ratio of electron and negative ion current I e /I H- was 9.1, with the RF power of 50 kW and the bias voltage of 29 V. The extracted H − current saturated with RF power, while the co-extracted electron current increased. The current experimental results indicated the HUNTER source was operated on a right trace and is ready for the surface mode operation with Cs seeding.

Efficiency investigation of a negative hydrogen ion beam production with the use of the gasdynamic ECR plasma source

Negative hydrogen ion sources are of great demand in modern physics as injectors into accelerators and drivers for neutral beam injectors for fusion devices. It has been shown earlier that the use of the gasdynamic ECR discharge provides the opportunity to extract up to 80 mA/cm2 of negative ion current density. We studied experimentally the volumetric negative hydrogen ion production and vacuum ultraviolet emission in a gasdynamic ECR discharge. The high-density plasma was sustained by the pulsed 37 GHz / 100 kW gyrotron radiation in a magnetic configuration consisting of two consecutive simple mirror traps. The future prospects of the volumetric H− source based on the gasdynamic ECR discharge related to the transition from pulsed to continuous operating mode with the use of an improved magnetic confinement system are discussed. Numerical simulation of the negative hydrogen ion beam extraction at the continuous operating mode facility was performed. Optimal configuration of the extraction electrodes and the electron damping magnets was found.

Breakdown Oscillations of the ISIS H− Penning Ion-Source Hydrogen–Cesium Discharge

The ISIS Penning ion source is a pulsed dc discharge surface plasma source delivering a 55-mA beam of negative hydrogen ions (H−) in 250- ${ mu }\text{s}$ pulses to the ISIS accelerator for neutron and muon physics at 50-Hz pulse repetition rate. The H− ions are extracted from the hydrogen–cesium discharge sustained by a current-regulated pulsed power supply delivering a current of 50–60 A. The discharge exhibits an ignition transient that requires the pulselength to be prolonged to 700 ${ mu }\text{s}$ to extract a clean H− beam pulse. The current and voltage fluctuations during the breakdown oscillation enhance the sputtering damage of the ion-source cathodes, thus contributing to the lifetime limitation of the device. It has been hypothesized that the breakdown oscillations are due to cesium dynamics causing the surface work function and electron emission from the cathode surfaces to fluctuate periodically. Experiments with the ISIS H− Penning ion source, reported here, reveal that the breakdown oscillations are primarily driven by the discharge power supply and its auxiliary circuits, whereas cesium dynamics have only a minor effect on the ignition transient. The conclusion is derived from careful characterization of the discharge current–voltage characteristics, optical emission measurements combined with a 0-D model, and comparison of different discharge geometries. Engineering solutions to mitigate the breakdown oscillations are presented and the potential effect of the applied modifications on the molybdenum cathode erosion by cesium sputtering is estimated.

Operation of RF-driven negative hydrogen ion source in China Spallation Neutron Source.

The China Spallation Neutron Source started delivering neutron beams to users in March 2018. To upgrade the beam power to 500 kW and improve the performance of the ion source, an RF-driven negative hydrogen (H-) ion source is under development. The source has a silicon nitride ceramic plasma chamber surrounded by a 4.5-turn antenna. The plasma is ignited by a pulsed DC spark gap and then driven by a 2 MHz solid-state amplifier with a repetition rate of 25 Hz. The commissioning of the source started in January 2019. When uncesiated, it produced about 20 mA beam at an RF power of 32 kW and pulse width of 450 μs. This paper describes the configuration of the ion source, several peculiar technologies used in it, and the first negative hydrogen (H-) ion beam extraction.

Physics and engineering design of 400 keV H- accelerator for negative ion based neutral beam injection system in China.

A research project of the China Fusion Engineering Test Reactor (CFETR) Negative ion-based Neutral Beam Injection (NNBI) prototype has been started in China. The objectives of the CFETR NNBI prototype are to produce a negative hydrogen ion beam of >20 A up to 400 keV for 3600 s and to attain a neutralization efficiency of >50%. In order to identify and optimize the design of the negative ion accelerator, a self-consistent model has been developed to consider all key physics and engineering issues (electric and magnetic fields, background gas flow, beam optics, beam-gas interaction, secondary particle trajectories, power deposition on grids, heat removal design, and mounting pattern). This paper presents the primary results by applying the self-consistent model to the current design of the 400 keV H- accelerator of the CFETR NNBI prototype.